ppc4xx: Fix NAND booting targets after 4xx linker script consolidation

Somehow I missed the NAND booting targets in the 4xx linker script
consolidation patchset. This patch fixes this issue.

Signed-off-by: Stefan Roese <sr@denx.de>
diff --git a/board/amcc/bamboo/config.mk b/board/amcc/bamboo/config.mk
index a37636a..72b6bc0 100644
--- a/board/amcc/bamboo/config.mk
+++ b/board/amcc/bamboo/config.mk
@@ -36,3 +36,7 @@
 ifeq ($(dbcr),1)
 PLATFORM_CPPFLAGS += -DCONFIG_SYS_INIT_DBCR=0x8cff0000
 endif
+
+ifdef CONFIG_NAND_U_BOOT
+LDSCRIPT := $(TOPDIR)/board/$(BOARDDIR)/u-boot-nand.lds
+endif